In recent years, the emission control regulations imposed by various regulatory bodies make LNG an attractive marine fuel and substitute for diesel powered onshore power plants; thus, demand for LNG bunker barges and small scale (break-bulk) LNG carriers has been increased significantly. In addition, due to widening of Emission Control Areas (ECAs) and implementation of 0.5% Sulphur cap limits by 1 Jan. 2020, LNG becomes an attractive fuel alternative for harbor vessels including tugs.
All these ocean-going LNG fueled carriers or harbor tugs contain either LNG cargo containment system, or LNG fuel tank to supply natural gas fuel for propulsion and other onboard electricity demand. Heat ingress into the cargo containment system or LNG fuel tank vaporizes some of the liquid to generate boil-off gas (BOG), which eventually increases the tank pressure. Regulations prohibit venting of excess BOG and marine class societies have mandated to have shipboard BOG management system. Onboard consumption of BOG as ship fuel is not an ideal solution since it could lead to deteriorate the original Wobbe index of the cargo as BOG is rich with nitrogen comparing to the LNG cargo composition. Thermal oxidation of the excess BOG is one of the available options, but it would be the costliest alternative.
Reliquefaction of BOG will overcome the above-mentioned issues. U.S. Pat. No. 3,874,185 discloses one conventional approach that it utilizes a closed loop nitrogen refrigeration. The problem with this conventional approach is that it requires large reliquefaction plant comprising a compressor and expander, which leads to higher capital cost and larger footprint.
U.S. Pat. No. 8,739,569 teaches a process to address the problems associated with Brayton cycle, which also utilizes nitrogen as a refrigerant. Instead of Brayton cycle, it introduces a plurality of pulse-tube refrigerators with secondary—refrigerant, to condensate BOG by vaporizing the liquid nitrogen (secondary refrigerant). A pulse tube refrigerator could be smaller than conventional Brayton cycle, but it is not a cost-effective approach due to the number of refrigerators required to perform the same thermal duty.
U.S. Pat. No. 3,857,245 describes another approach by utilizing the natural gas as a working fluid operate in an open cycle. In this process, partially condensed BOG can be obtained with typically 30 percent of liquid phase formation. This could be the simplest form of BOG reliquefaction (partial) system, but the remaining 60 to 70 percent of non-condensed BOG has to be sent to a burner for combustion. It makes the system inefficient and limiting the application on shipboard vessels.